Frequent tipster Danny Korem provides a method of drawing a polyline in AutoCAD by snapping to objects that have elevation, but keeping the polyline's elevation at zero.
"While designing, architects cross-reference surveyors' maps and layouts. Mostly these CAD files are products created by sophisticated equipment and all of the points have z values; contour lines have their respective elevation values, and so on.
"While 2D drafting, we would typically like to get rid of the 3D data. We can deal with objects and change their properties, but should we? It's the essence of x-referencing not to intervene in your design team’s data. And what about 3D data we can't deal with? (For instance, changing the z insertion point of a block won't necessarily alter the z values within it.)
"I opted for a shorter and simpler procedure. I used the CUI (Customize User Interface) command to create the following new command:
^C^C(command "pline" ".xy" (getpoint) "0")
For practical reasons I used the Polyline (Pline) command, since the elevation is set by the first point you pick while object snapping to a point. I use the .xy filter of the point osnapped and use 0 as the z value. That way, no matter what the z value is, the pline's elevation will be kept to 0'. Finally, after assigning it a proper icon, I dragged the command to the quick-access toolbar. Though plines became very flexible (hot grip add vertex and mid-grip turn to arc), one can still explode the polyline into lines and arcs but all of them have start, end, and center z points with value=0."
Notes from Cadalyst Tip Reviewer Brian Benton: This works, and it is very useful. The macro in the new command forces your polyline to be drawn at an elevation of zero units.
In the past, I would trace over the objects with a Z value and use the Flatten command. From time to time, however, there would be side effects that left my new linework unusable, or at least made it frustrating to use.